Cathodic protection system



Oct. 4, 1949. w. F. BONNER 2,483,397

CATHODIC PROTECTION SYSTEM Filed Aug. 13, 1945 6 *4 AC SUPPLY TRACK INVENTOR WALTER I. BONNER Patented Oct. 4, 1949 CATHODIC PROTECTION SYSTEM Walter F. Bonner, Glen Ridge, N. J assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application August 13, 1945, Serial No. 610,643

4 Claims. (Cl. 171-316) The present invention relates to a system for protecting underground metallic structures against electrolytic corrosion and more particularly to the so-called cathodic system of electrolytic corrosion prevention in which the potential of the buried structure is depressed below that of the surrounding earth.

Electrolytic corrosion of underground metallic structures such as building foundations, piping and electric cable sheaths, is occasioned by the presence in such structures of electric currents. These arise in some cases from a galvanic action between the metallic structure and the surrounding soil in which it is embedded, due to the chemical nature of the soil and the fact that the surrounding earth, particularly when moist, is a conductor of electricity. The currents within the underground metallic structure also commonly are caused by stray currents from such sources as electric railway tracks which are employed as one of the conductors for the supply of energy to the railway rolling equipment. When the conditions of relative potential between the metallic structure and the surrounding earth are such that current flow from the structure to the earth is set up, there results a gradual removal of metal from the structure with consequent corrosion and progressive weakening of the structure.

The tendency of underground metallic structures to undergo electrolytic corrosion can be overcome by the cathodic system of protection. This system is based on creating and maintaining a difference of potential between the underground metallic structure and the immediately surrounding earth such that the metal structure is at a lower potential than the earth, wherefore current flow from the metal structure to the earth is substantially prevented. Such systems commonly provide the desired difference of potential between the underground metallic structure and the surrounding earth by connecting the negative side of a rectified alternating current circuit to the buried structure. The positive side may be connected to a suitable electrode buried in the earth nearby, or in the case when the principal cause of the electrolysis is a nearby electric railway, to the railway track. In some instances the positive connection is made to the negative bus bar at the generating station.

To prevent the undesired electrolytic corrosion, the current required varies with soil conditions, but, ordinarily, currents of the order of amperes have proven adequate in practice. As a practical matter, however, the current flow may fluctuate due to various changes in the conditions such as changes in the conductivity of the soil due to rainfall and to changes in the potential of the track work of the railway. Rise of the current above values which are adequate to substantially prevent electrolytic action represents'an expenditure of energy which does not produce a corresponding beneficial result. Also, such large currents cause unnecessary heating and other undesired effects in the rectifiers.

It is an object of the present invention to provide an electrolytic corrosion control system in which the current flow under the influence of the corrosion control voltage is maintained below a predetermined maximum value.

A further object is to provide an electrolytic corrosion control circuit which is automatically responsive to rise of current above a predetermined value to limit the current flow.

In accordance with theinvention, the electrolytic corrosion control system includes a circuit connected to the underground structure which includes means for producing a difference of potential to depress the potential of the structure and means responsive to the value of the current in the circuit connected to the structure to render inoperative the means for producing the difference of potential.

In general, the system of the present invention includes a circuit connecting the underground metallic structure with the earth adjacent the structure, with the rails of an electric railway in the vicinity of the underground metallic structure or with the negative bus bar at the generating station. An electromotive force is impressed on this circuit in a direction to tend to cause current flow from the underground structure to the earth, the track, or the bus bar, as the case may be. Preferably, the current supply in practice is an alternating source which is connected to the circuit by means of a transformer whose secondary winding is in series with the circuit, in which case a suitable rectifier is provided in series with the transformer secondary winding. A shunt circuit is arranged across the rectifier and the transformer secondary winding. The shunt circuit contains a set of normally open relay contacts which are adapted to be closed by a relay operating winding, connected in series with the transformer secondary winding and the rectifier, when the current in the circuit exceeds a predetermined value. The relay may be provided with a second set of contacts arranged to open the supply circuit to the transformer primary winding simultaneously with the short cir- 3 cuiting of the rectifier and the transformer secondary winding.

The invention will be further understood from the following specification when read in connection with the annexed drawing, in which the single figure is a circuit diagram of an electrolytic corrosion control system embodying the inention.

For the purpose of illustration, a system adapted for the, protection of underground metallic pipe from electrolytic corrosion due to stray currents from an electric railway has been selected, but it will be understood that this is only one illustrative embodiment of the invention, which is also applicable to the protection of crosscountry pipe lines or sheathed electric cables in regions where no electric railway is located in the vicinity of the pipe line or cable. In this latter case, it will be appreciated, the cause of the electrolytic corrosion is the galvanic action between the underground structure and the adjacent soil.

In the accompanying drawing the underground pipe which is to be protected is illustrated at I. A rail of an adjacent electric railway track is illustrated at 2. In the absence of some system of protection, the pipe I would be gradually eroded by the action of stray currents from the track 2 which would flow into the pipe at points at which the pipe is of lower potential than the adjacent earth and would then leave the pipe at points where the potential of the adjacent earth is less than the potential of the pipe. To prevent this action a negative potential with respect to the earth is impressed on the pipe from a suitable source of alternating electromotive force 3. Alternating current from the source 3 is supplied to the primary winding 4 of a transformer 5 whose secondary winding 6 is connected between the pipe I and the track 2 by a circuit 8. A rectifier i is included in the circuit 8 in series with the secondary winding 6 of the transformer 5. The rectifier is so arranged as to permit current now in the direction from the pipe i to the track 2. It will be appreciated that the rectifier illustrated symbolically at I may be of any suitable type and may be a single rectifier or a set of rectifiers connected in the well-known Wheatstone bridge arrangement.

To prevent current flow through the circuit 8 under the influence of the voltage from the source 3 from rising above a predetermined value, a shunt circuit 9 is provided, connected in parallel with the rectifier I and the secondary winding 6 of the transformer. A relay in has its operating winding ll connected in the circuit 6, in series with the transformer secondary winding 6 and the rectifier 1, to be responsive to the current in this circuit. Relay III is provided with a set of normally-open contacts I! connected in the shunt circuit 9 which are adapted to be closed upon operation of the relay to close the shunt circuit 9. This operation will occur when the current flowing in circuit 8, and consequently through the operating winding H of relay I0, exceeds a preselected value which is determined by the characteristics of the relay l0.

Upon operation of the relay III and the consequent closing of the shunt circuit 9, the alternating current source and the rectifier are for all practical purposes rendered inoperative and the connection between the pipe and the track is a direct grounding connection through the shunt circuit 9. when the current in the circuit I falls below the predetermined maximum, the relay it opens the shunt circuit 9.

I claim:

1. A cathodic protective system for protecting underground structures against electrolytic corrosion, including, in combination, an underground structure, an electric railway track in the vicinity of the structure, a circuit connecting the track and the underground structure, means for impressing an electromotive force on said circuit, and means for rendering said first-named means inoperative upon rise of current in said circuit above a predetermined value, and operative when said potential difierence falls below said value.

2. A cathodic protective system for protecting underground structures from electrolytic corrosion, including, in combination, an underground structure, a grounding circuit connected to said structure, a source of electromotive force connected in series in said circuit, and means for short circuiting said source of electromotive force upon rise of current in said circuit above a predetermined value.

3. A cathodic protective system for protecting underground metallic structures from electrolytic corrosion, including, in combination, an underground metallic structure, an electric railway track in the vicinity of the structure, a circuit connecting the track and the underground structure, a rectifier and a source of alternating electromotive force connected in series in said circuit and means for short circuiting said rectifier and source of electromotive force upon rise of current in said circuit above a predetermined value.

4. A cathodic protective system for protecting underground metallic structures from electrolytic corrosion, including, in combination, an underground metallic structure, an electric railway track in the vicinity of the underground structure, a circuit connecting the track and the underground structure, a rectifier connected in said circuit, a source of electromotive force, a transformer having a primary winding connected to said source and a secondary winding connected in said circuit in series with said rectifier, a relay having an operating winding connected in said circuit in series with the rectifier and the transformer winding, a pair of normally-open contacts connected to said circuit in shunt with said rectifier and transformer secondary winding, and a set of normally-closed contacts connected in the supply circuit to said transformer primary winding, whereby said relay operates upon rise of current in said circuit above a predetermined value to short circuit the rectifier and transformer secondary winding and to disconnect said source of electromotive force from the transformer primary winding.

WALTER. F. BONNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Bagwell Mar. 5, 1946 

